It's up to you, but this doesn't sound like a healthy relationship for you. Personally I see no reason why you and boyfriend shouldn't get to have sex just the two of you, especially if she *is* allowed to have sex alone with him, and balks when it's all three of you. Where does that leave you? It seems very unfair.
I would tell them that if you are going to stay, then they are going to have to adjust some of the unfair rules. And maybe your boyfriend should open his mind a little about letting this thing be a V, if that's what it needs to be.
It's obviously not working the way things are; something's got to change.
I hope you find an answer to your dilemma and get some more peace in your life. I am always here to help if I can.
Love means never having to say, "Put down that meat cleaver!"